1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit suitable for a large sized LCD device having improved uniformity of luminance.
2. Discussion of the Related Art
A cathode ray tube (CRT), has been widely used for computer monitors and televisions. However, the CRT is heavy and bulky. Thus, display devices, such as a liquid crystal display (LCD), a plasma display panel (PDP) and an electroluminescene display (ELD), have been substituted for the CRT. Among the alternative display devices, the LCD device has been very popular due to the LCD device having a low power consumption, thin profile and lightness in weight. The LCD device has been employed as a monitor for a desktop computer, a large sized television display device, as well as for a laptop computer.
Most LCD devices control the light transmittance from ambient light to display images. In this respect, it is necessary to form an additional light source, such as a backlight unit in an LCD panel. In general, the backlight unit, used as the light source of the LCD device, has a cylindrical fluorescent lamp, which is classified into two types, a direct type and an edge type, according to a disposition of the fluorescent lamp.
In the edge type backlight unit, a lamp unit is provided at one side of a light-guiding plate. The lamp unit is provided with a lamp emitting light. A lamp holder is inserted into both ends of the lamp to protect the lamp. A reflective plate has one side inserted to the light-guiding plate and surrounds the circumference of the lamp, to reflect the light emitted from the lamp to the light-guiding plate. The edge type backlight unit is generally applied to relatively small sized LCD devices, such as the monitors of the laptop computer and the desktop computer. The edge type backlight is advantageous in that it has great uniformity of light, long life span, and a thin profile.
The direct type backlight unit is suitable for a large sized LCD device of 20 inches or more. The direct type backlight unit includes a plurality of lamps arranged in one direction below a light-diffusion plate to directly illuminate an entire surface of the LCD panel with light. The direct type backlight unit has great light efficiency and is commonly used for the large sized LCD devices requiring high luminance, such as televisions.
A related art backlight unit for an LCD device will be described with reference to the accompanying drawings. FIG. 1 is a perspective view of a direct type backlight unit, according to the related art. FIG. 2 is a plane view of the arrangement of fluorescent lamps in a direct type backlight unit, according to the related art. FIG. 3 is a plane view of the arrangement of fluorescent lamps in a direct type backlight unit, according to another embodiment of the related art.
As shown in FIG. 1 and FIG. 2, the backlight unit according to the related art includes a plurality of fluorescent lamps 1, an outer case 3, and light-scattering means 5a, 5b and 5c. The outer case 3 fixes and supports the plurality of fluorescent lamps 1, and the light-scattering means 5a, 5b and 5c are provided between the fluorescent lamps 1 and an LCD panel (not shown). The light-scattering means 5a, 5b and 5c prevent the silhouette of the fluorescent lamps 1 from being reflected on a display surface of the LCD panel (not shown), and provide a light source with uniform luminance. For improving the light-scattering effect, the light-scattering means includes a diffusion plate 5a, a prism sheet 5b, and a protection sheet 5c. Also, a reflective plate 7 is provided inside the outer case 3 for concentrating the light emitted from the fluorescent lamps 1 to the display part of the LCD panel.
Each fluorescent lamp 1 is a cold cathode fluorescent lamp (CCFL) 1. Electrode parts 2a and 2b are provided at both ends in a tube of the CCFL, and power supplying wires 9a and 9b are respectively connected with the electrode parts 2a and 2b. Both ends of the fluorescent lamp 1 are inserted into and fixed to both sides of the outer case 3. In this state, as a voltage is applied to the electrode parts 2a and 2b, through the power supplying wires 9a and 9b, the fluorescent lamp 1 emits light, and the fluorescent lamp 1 provides light for the LCD device. Although not shown, the voltage applied to the power supplying wires 9a and 9b is provided from an inverter, wherein the inverter is provided at the rear of the outer case 3.
In the direct type backlight unit, according to the related art as shown in FIG. 2, the fluorescent lamps 1 are provided at fixed intervals and have a length corresponding to a horizontal line of a light-emitting surface of the backlight unit. Accordingly, the length of the fluorescent lamp 1 increases with an increase in the horizontal length of the light-emitting surface of the backlight unit. For example, if the LCD device has a size of 30 inches to 40 inches, it is necessary to employ fluorescent lamps having a length of 700□ or more.
As the length of the fluorescent lamp increases, the fabrication process of the fluorescent lamp becomes more difficult. Also, when a relatively long fluorescent lamp is disposed in the backlight unit, the assembling process is complicated. Furthermore, the backlight unit is more susceptible to damage by external forces. In addition, when driving the fluorescent lamp, it is difficult to realize uniform luminance. Also, it is required to maintain a high driving voltage, which may lead to the problem of electro magnetic interference (EMI) by an electric interference of a driving circuit for obtaining stability and driving the LCD panel.
To solve these problems, a different design in a direct type backlight unit, according to the related art, is shown in FIG. 3. As shown in FIG. 3, fluorescent lamps 30, each having a length corresponding to a vertical line of the light-emitting surface of the backlight unit, are provided at fixed intervals along a length direction of the light-emitting surface of the backlight unit. Electrode parts 32a and 32b are formed at both ends of a tube in each fluorescent lamp 30, and power supplying wires 33a and 33b are connected with the electrode parts 32a and 32b. The backlight unit also includes a reflective plate 31.
However, as shown in FIG. 3, if the fluorescent lamps 30 extend in the vertical direction of the light-emitting surface of the backlight unit, hydrargyrum moves in the tube of the fluorescent lamp 30 downward. This movement of the hydrargyrum deteriorates the characteristics of the fluorescent lamp 30. As a result, it is impossible to obtain uniformity of luminance.